The dark angel and the heart, both consisting of volcanic sands, are located in the southern polar region of the planet, not far from the polar cap, at approximately 78 degrees south. Currently it is summer at this location. The permanent ice cap, consisting mostly of water ice and frozen carbon dioxide, now has a diameter of 400 kilometres and an average thickness of 1.5 kilometres. These are dimensions that are roughly comparable to the ice-covered island of Greenland on Earth. However, it is only this size in the southern summer. During the six-month winter, the south pole ice cap on Mars extends further, to almost 60 degrees south. Even in summer in the southern hemisphere of Mars, temperatures never rise above zero degrees Celsius. In winter, temperatures down to minus 133 degrees Celsius cause the carbon dioxide from the atmosphere to freeze and fall onto the ice cap as snow. This blanket of carbon dioxide ice is between one and two metres thick and sublimates again with the next spring, thus evaporating and re-exposing the landscape. Only on the permanent ice cap of the south pole does a thin layer remain. The atmosphere on Mars contains very little water vapour that can freeze to form ice. Water vapour forms only 0.02 percent of the planet's gas envelope, which consists mainly of carbon dioxide and nitrogen; on Earth, there is twenty times more water vapour in the atmosphere, averaging a concentration of 0.4 percent. Therefore, the angel and the heart are only visible in the southern summer; in winter they lie hidden under the layer of frozen carbon dioxide.